collie: add -f option for force vdi rollback

"collie vdi rollback" does confirmation before actual rollback. The -f
option for skipping it should be supported for external programs which
use collie as a backend.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>